Ink jet printers apply a wide range of ink compositions to a wide range of surfaces as discrete droplets of ink which form an image on the surface. The droplets can be applied using drop on demand or impulse jet printers in which individual droplets are ejected from individual nozzle orifices in an array of nozzles, the position of the printed droplet on the surface being dictated by the nozzle in the array from which it is ejected. Alternatively, the ink jet printer can be a continuous jet printer in which droplets are formed from a jet of ink issuing from a single nozzle, the droplets being given an electric charge and passing through a deflection field which deflects the flight of the individual droplets away from the straight line path by an amount depending upon the strength of the charge and/or the deflecting field. For convenience, the term ink jet printer will be used herein to denote all such types of printer.
The inks can be used to form a wide range of types of image, for example date, batch or quality codes, bar codes, manufacturer's names, logos or other images, both pictorial and alphanumeric. Furthermore, such images need not be visual, but may be machine readable or detectable, for example by electo-optic means when the image is caused to fluoresce under infra-red or ultra-violet radiation or when the pigment is electromagnetic and the image carries electromagnetic data. For convenience, the term image will be used herein to denote in general all such images.
Ink jet printers employ a wide range of inks which, in general comprise a pigment, dyestuff or other image forming component and a resin or other binder in a fluid carrier medium. The fluid carrier medium can be water or an organic solvent or carrier, for example an alkanol, ketone or ester and mixtures thereof. Where the ink is to be used in a continuous jet printer, the ink has a conductivity so that it can accept an electric charge. This conductivity can be provided by salts or other polar materials present in one or more of the above ingredients, or can be provided by one or more additional materials.
It has been proposed to print images on hard gelatin capsules using ink jet printers. Since such capsules usually contain a material which is to be ingested by the user or applied to the skin, the inks used should contain only physiologically acceptable ingredients. However, where such inks are applied to soft gelatin surfaces as opposed to hard gelatin surfaces, it has been found that the gelatin sweats and the printed droplets readily detach from the surface so that the printed image is destroyed. As a result no commercially acceptable method exists for printing images on soft gelatin surfaces.
Surprisingly, we have found that the above problems can be reduced by the use of shellac, notably ammoniated shellac, as the binder in inks applied to soft gelatin surfaces.